Concentric conductor system



Nov. 11, 1930. H. A. AFFEL CQNCENTRIC CONDUCTOR SYSTEM Filed May 23,1929 2 Sheets-Sheet 1 22 22. 4,2186% mud v INVVENTOR Z BY .7

75 ATTORNEY Nov. 11, 1930.

H. A. AFFEL GONCENTRIC CONDUCTOR SYSTEM Filed May 25, 1929 '2Sheets-Sheet 2 6' okezi 'electrz'c Weatherproof Spiral Covering INVENTORE 419 766 [In q 7?! a ATTORNEY Patented Nov. 11, 1930 ATN osF cr.

HERMAN A. AFFEL, OF RIDGEWOOJD, NEW JERSEY, ASSIGNOR TO AMERICANTELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK CQNCENTRICCONDUCTOR SYSTEM Application filed May 23, 1929. Serial No. 365,523.

This invention relates to a conducting system for transmitting withsmallattenuation a band of frequencies whose upper limit extends wellabove-the frequencies now employed in carrier transmission.

Modern developments in the art of communication render it highlydesirable to have available for transmission purposes a system whichwill transmit without undue attenuation frequencies extending from theaudio frequency range well up into the radio frequency range. Forexample, high grade circuits are now required for the transmission ofprograms over telephone lines to broadcasting stations. In order totransmit musical programs it is necessary to provide circuits that willtransmit a band of frequencies extending well up toward 10,000 cycles,as compared with the voice range ordinarily employed in telephony, whichdid not exceed 2,500 cycles. For the best quality of transmission ofmusic it might be desirable to transmit over telephone lines frequenciesup to the audio limit, which would be in the neighborhood of 15,000 or20,000 cycles. Modern cable circuits are not ordinarily adapted fortransmission of such high frequencies, and the only commercial circuitnow available which would be capable of transmitting frequencies of thisorder would be the open wire circuits which have heretofore beenemployed for high frequency multiplex carried transmission.

Even for carrier transmission, open wire lines have been founduncommercial for the transmission of frequencies much above 30,000cycles. If, therefore, an open wire line is used for the transmission ofa high grade audio frequency program involving frequenciesin theneighborhood of 15,000

or 20,000 cycles, the remanent frequency range above such audio bandwould be so narrow as to be of little use for carrier transmission. Fromthis standpoint, therefore, it would be highly desirable to haveavailable a circuit which would transmit without undue attenuationfrequencies much higher than 30,000 cycles. 1

The modern development of television also introduces a new factor.Existing television systems which have been experimented with have beenlimited to the transmission of a small image of a few square inches inarea, and in such image the elements of the picture making up the entireimage have been relatively large, so that the picture is not welldefined. The mechanical problems involved in designing a televisiontransmitting and receiving apparatus capable of picking up and receivingwith excellent definition a representation of some large scene, such asa ball game or a theatrical performance, are readily capable of solutionby known means, but the transmission of such a picture electricallyinvolves the transmission of frequencies from zero up to theneighborhood of 500,000 cycles, and there is no telephone circuit nowavailable which would commercially transmit any such range offrequencies because of the enormous attenuation involved at frequenciesabove about 30,000 cycles. It therefore becomes desirable to haveavailable a conducting system which would transmit without undueattenuation a wide range of frequencies including the extremely highfrequencies necessary for television, the circuit being at the same timeavailable if necessary for the transmission of a very large number ofcarrier channels or for any desired number of bands of sufficient widthfor the transmission, without undue distortion, of high grade programs.

In accordance with the present invention, acircuithaving these desirablecharacteristics is obtained by employing concentric conductors ofrelatively large diameter, one acting as a return for the other. The twoconductors may be insulated from each other and held in properconcentric relationby means of a spiral insulating arrangementcomprising a series of spokes passed trans-' versely through the innerconductor and arranged spirally along the inner conductor with theirouter ends acting as supports for their outer conductor. Preferably theinner conductor, and, if desired, both conductors, may be constructed ofa large number of fine wires of conducting material insulated from eachother by some suitable insulating enamel and braided together to form bemade waterproof, with theresult that the leakage losses between theconductors (which in the case of ordinary open wire construction varygreatly with weather conditions and at high frequencies contribute verysubstantially to'the attenuation) may be made small and constant.Furthermore, the increase in conductor resistance with frequency due tothe skin effect is relatively small, so

that the increase in the attenuation component due to resistance is muchless rapidthan for ordinary open wire construction. Also, the form ofconstruction is such that 5 interference from nearby circuits and noisecoming from external sources will be practically negligible. Moreover,the nature of the circuit is such that even though the outer conductorbe grounded it will not be subject to interference from ground currents.This 7 enables the conductor to be laid directly on the metallicsupports of an overhead cable system or underground in aconduit,without'any external insulation. Also the velocity oftransmission will be substantially uniform for all frequencies.

The invention will now be more fully un-- derstood from the followingdescription when read in connection with the accompanying drawing, ofwhich Figures 1 to 5 invention and Fig. 6 is abroken section of aportion of the conducting system in accordance with the presentinvention.

Referring to Fig. 6' of the drawing, 10 designates an outer conductorwhich may be in the form of a thin-cylindrical shell constructed eitherof solid conducting material or of a large number of fine wires ofconducting material, each insulated from the other by a suitable enamelcoating and woven or braided together. A second conductor 12 is mountedconcentrically withthe outer conductor 10, the inner conductor likewisebeing constructed of braided fine wires insulated from eachother byenamel or other insulating coating. One of the conductors acts as areturn for the other and not 'as a mere shield, this fact beingindicated by the conventional representation of a source, of alternatingelectromotive force G with its terminals connected to the twoconductors. i i In order that the attenuation may be small at highfrequencies, the leakage lossbetween the conductors must be as small aspossible.

are curves illustrating the principles of the.

passed transversely through openings or interstices between the Wires ofthe inner conductor 12, and the successive spokes being arrangedspirally along the inner conductor with their outer ends extendingoutwardly to support the outer conductor; The successive spokes 14should be separated as far as possible from each other and the pitch ofthe spiral in which the spokes are arranged should be made as large aspossible consistent with mechanical strength. The spokes should also becomposed of some dielectric material of small loss an 1e and lowdielectric constant since if these conditions are obtained, the leakageloss (which in the ordinary open wire-system comprises a large part ofthe attenuation) may be made so small as. to be practically negligible.

Pyrex glass or isolantite is asuitable niater'ial for the spokes 14.

In this connection it should be noted that as the outer shell may bemade watertight by a suitable waterproof cover 16, the insulating spokes14 will be maintained dry and free from dirt or contamination so thatthe leakage loss will not increase or change with time. In ordinary openwire construction where the insulators are exposed to the air and to theaction of the elements, the insulators become coated with a film ofrelatively high resistance conductive material which introduces largeleakage losses, and these leakage losses are enormously increased whenthe external surfaces of the insulators become wet. If it were possibleto maintain an open wire line with itsinsulators in the dry and cleancondition which characterizes them when they-come from the factory, theattenuation component due to leakage would be so small as to benegligible. It will therefore be apparent that even if the dielectricmaterial of which the spokes 14 is composed is not the most perfect fromthe standpoint of low loss angle and small dielectric constant, theattenuation due to the insulating member may still be practicallynegligible. I

As will be explained later, a conducting system of this type will bepractically free from external interference even though the outerconductor is grounded. Since, therefore, it is possible to mount theconcentf'ic conductor arrangement upon the metallic supportof anordinaryoverhead cable construction or to permit the arrangement to beburied directly in the cum]. or laid in a conduit such as mighteemployed for underground cable, the flexible construction of the systemdue to the braided arrangement of the conductors and to the spirallymounted insulating supports of spokes especially adapts it'for mountingin overhead cable hangers or in underground cable conduit.

, In the ordinary type of conductor system, either open wire or cable,where one solid wire acts as a return for another solid wire, thecomponent of the attenuation which is due to the skin effect is of greatimportance at high frequencies. As is well known, where a solidconductor is employed, as the frequency becomes higher more and more ofthe current tends to flow at or near the surface of the conductor, sothat the conductive material near the center of the conductor takes butlittle part in the action at high frequencies. As a consequence, theconductor resistance increases with frequency as a smaller and smallerpart of the cross-section of the conductor is usefully employed. If thesame amount of conductor material is arranged in the form of arelatively thin shell, the resistance at any given high frequency isvery much reduced because now more nearly all of the material of theconductor is usefully employed in transmitting current. In a system ofconcentric conductors, such as described in connection with the presentinvention, if both conductors are in the form of thin hollow shells ofconducting material, the conductors offer much less resistance at highfrequencies due to' the skin effect for the same amount of conductivematerial than in the case of an ordinary transmission circuit consistingof two solid wires. In fact, in a concentric conductor system, thecurrent at higher frequencies tends to flow more and more at the innersurface of the outer conductor and at the outer surface of the inner If,however, two cylindrical conducting shells are formed of a large numberof fine wires insulated from each other but braided together so that thewires loop in and out toward and from the common center of the twoconductors, the current can no longer follow along the adjacent skins orsurfaces of the two cylinders but is compelled to follow the wires ofwhich the cylinders are com.- posed, with the result that both theconducting material adjacent the inner surface of the outer cylinder andthat at the outer surface of the outer cylinder will'be used for.conducting the current at high frequency, and the same holds true forthe inner conductor. This accordingly reduces the skin effect verymaterially with the result that while the component of the attenuationwhich is due to the conductor resistance may increase to some extentwith frequency, the rate of increase is very much less than in the caseof an open wire line.

By means of the construction above described, therefore, I have theonecomponent of the attenuation which is due to leakage losses or theso-called shunt effect reduced to practically negligible proportions byreason of the fact that the dielectric between the conductors is verylargely of air In order to understand this more clearly it should beremembered that the interference between any two circuits is due to thefact that the one circuit lies within either the electric field or themagnetic field or both, of the other circuit. Considering first themagnetic field, let us consider two conductors a and b circular incrosssection and arranged side by side, one acting as a return for theother. These conductors are shown in section in Fig. 2. The lines offorce due to the magnetic field surround each conductor and are crowdedtogether in the space between the two conductors. Any other conductingsystem introduced at. a point where the conductors of such other systemwill be cut by these lines of force will have induced therein cross-talkfrom the conductor system a b. If, now, we have two conductors 10 and12, as shown in Fig. 1, in the form of hollow shells concen tricallyarranged and the one acting as a return for the other, each conductorhas lines of magnetic force surrounding it, each successive line offorce being of larger radius and all of the lines, due to the currentflowing in the particular conductor, such as 12, being external thereto.As the current flows in one direction through the conductor 12 and inthe opposite direction through the conductor 10, the lines of magneticforce due to the current through the conductor 12 are in one direction,as indicated by the arrows, while those due to the current flowing intheconductor 12 are in the opposite direction. Now, an inspection ofFig. 1 shows thatsome of the lines of force due to the current in theconductor 12 are within the conductor 10, but none are within theconductor 12. On the other hand, all of the lines of force due to thecurrent fiowing in the conductor 10 are external to said conductor, andthe two magnetic fields produced by the currents flowing in the twoconductors tend to oppose each other outside of the conductor 10. Theresultant field of magnetic force external to the conductor 10 is,therefore, very small, and the only eflective magnetic field lies withinthe space between the two conductors. Since the external magnetic fieldis very small it is obvious that another conductive system external tothe conductor 10 will not receive any appreciable amount of cross-talkinterference from the conducting system 10-12.

In so far as the electric field is concerned, the distribution of thefield in the case of tWo parallel conductors a and b is as indicated inFig. 4, so that any external conductor which is cut by the lines ofelectric force between a and b will have cross-talk induced therein. Inthecase of the two concentric conductors 10-12, however, the electricfield set up due to currents flowing in the two conductors is entirelybetween the adjacent surfaces of the two conductors, as indicated inFig. 3. No external conductor can possibly be cut by any of thelines ofthe-electric field due to current flowing in the conductor 12 andreturning in the conductor 10, or vice versa, andhence so far as theelectric field is concerned, no possible external interference can takeplace.

The concentric arrangement not only has the advantage that it producessubstantially no external field to interfere in other circuits, but itis practically free from interference due to any external source. Forexample, referring to Fig. 5, let us assume that some external forceproduces a field asrepresented by the arrows. The lines of force cuttingthe two concentric conductors produce differences in potential betweentwo conductors. For example, consider the points 0 and d, the one on theouter surface of the. conductor 12 and the other on the inner surface ofthe conductor 10. The lines of force cutting the two conductors producean induced e. m. f. between these points'in the direction and having thevalue indicated by the arrow ,0 d. Since the same number of lines offorce out the two conductors on the opposite side of'the diagram, adifference in potential indicated by the arrow =c'--d will be producedbetwen the'two points 0' and do. The induced potential c-al, however,tends to produce a curent flow equal to and opposite that induced by thediifen. ence of potential at c'-d", so that a balance is obtained. 'Dueto the symmetry of the conducting system with respect to the cut tinglines of force, all differences in potential induced between any othertwo points of the two conductors will be balanced by similar diflerencesof potential induced at corresponding points on the opposite side, sothat if the interfering field is evenly distributed through thecross-sectional area of the conducting system (as would be the casewhere the interfering source is not too near the system) substantially.no interfering effect would result inthe conducting system 101-2.

While the foregoing explanation only applies to fields perpendicular tothe axis of oints of the the conducting system, field componentsparallel to the axis are also preventedfrom causing interference. Thisis because the skin effect in the outer conductor furnishes protectionagainst such fields.

As has been previously stated, the concentric conducting system is freefrom externalinterfence even though the outer conductor be grounded, andhence there is no necessity for insulating the outer conductor frommetallic supports in'case it is mounted like an overhead cable, or fromground in case it is placed in'a conduit. The reason for this is that aground return circuit is noisy, due to the fact that a wire supportedabove ground forms with the ground a loop to pick up stray fields. Butfrom the diagram of Fig. 5 it is evident that if the outer conductorsuch as 10 is grounded so that it in effect becomes a ground return forthe tube 12, it is only the space between the two concentric conductorsthat acts as the loop to pick up stray fields. Hence, as has been justexplained in connection with Fig. 5,'substantially no interferingcurrents are induced in the conductors 10-12. In order that a conductingsystem such as herein disclosed may have as small attenuaable that thesystem should be of such character that it might be used in existingcable .ducts or in connection with present aerial cable construction.For these reasons, in practice, 1t is convenient to make the diameiooter of the external conductor not much greater than about two andfive-eighths inches, if the conductoris to be used in the existingtelephone. plant. For economical reasons the thickness of the conductorsshould be made as'smallas is consistent with securing proper electricalcharacteristics and mechanical strength. In generalit-hasbeen'found'that f if the cylindrical conductor is made thickenough to satisfy the mechanical requirements,,the electrical resistanceis not a limiting factor in the attenuation atfhigh frequencies. This isdue to the skin effect, which, as previously-described: causes thecurrent to crowd to the outer'surface of-the inner conductor and theinner surface of the outer conductor as the frequency increases, therebyrendering the remaining cross-sectional area of little utilty forcarrying curent. I

As theouter conductor is, or at least can be made Watertight, theleakage losses can be reduced to veryvlow values by the use of pyrex orother insulation where mechanical support is necessary, with the largestpossible air space between the two conductors. Under these circumstancesthe leakage loss will not change with weather conditions. For zeroleakage (a condition which would represents the resistance, C thecapacity and L the inductance. From this expression it is evident thatthe values of R and C should beas small as possible. At high frequenciesR is inversely proportional to the diameter of the conductor, and hencethe attenuation will be smaller at any agiven frequency the larger thediameter of the conductor. The capacity C also is an inverse function ofthe diameter and decreases as the difference between the diameters ofthe inner and outer conductors increases. Consequently, if the diameterof the outer conductor is fixed, as the diameter of the inner conductorincreases from some small value the resistance of the conducting systemdecreases, while at the same time the capacity increases. The decreasesin resistance tends to reduce the attenuation, while'the increase incapacity tends to increase the attenuation. For a given diameter of theinner conductor these two eifects balance and the attenuation becomes aminimum.

At 500,000 cycles the attenuation per mile of a concentric conductorsystem as described above will be very much less than the attenuation ofa 165 gauge open wire circuit. There is a further advantage in using theformer on account of the lower levels to which the current may beattenuated before a repeater is necessary. This is due to the absence ofcoupling to external fields and results in a very low noise level. On an"open wire circuit the level could not be allowed to go below -50transmission units, while with the concentric conductor system it mightbe permitted to fall as low as '80 transmission units. If the repeatersare adjusted to give an output of +10 transmission units this wouldresult in a repeater spacing of thirty-six miles for the open wirecircuit and 210 miles for the concentric return circuit. It appears tobe impractical to devise a transmission system for an open wire circuitat such high frequencies, and thehigh frequency cross-talk would limitits use to one circuit on a given lead. Due to the absence of couplingsto other circuits this limitation would not apply to the tubularconductor system, and any desired number of such conductor systems mightbe mounted upon the same pole line or carried in adjacent conduitswithout undue interference.

It follows, therefore, for the transmission of frequencies up to 500,000cycles an open wire circuit would be quite unsuitable, whereas theconcentric conductor system would carry frequencies as high as 1,000,000to 2,000,000 cycles or higher, without undue attenuation.

A carrier telephone system could'be operated.

over such a conductor with as many as one to two hundred two-waychannels, allowing 5,000 cycles for each channel in each direction.

This is comparable to the numberof circuits which might be obtained fromthe pairs of wires in a cable of equivalent size. Any particular circuitin the cable could not be used for the transmission of frequencies muchabove the ordinary telephone range, and hence could not be employed forthe transmission of musical programs involving frequencies up to theaudio limit without using a very expensive system; It is impracticableto arrange a cable circuit to transmit frequencies high enough for goodtelevision transmission. The concentric conductor system, on the otherhand, may be employed for either program 7 transmission or television.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely difl'erent from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

- What is claimed is:

1. In a conductive system for the communication of intelligence, twoconcentric conductors of cylindrical form, and insulating means forseparating the conductors electrically and maintaining them inconcentric relation, said means comprising spoke-like members ofdielectric material passing transversely through the inner conductor sothat successive spoke-like members are arranged spirally along theconductor.

2. In a conductive system for the communication of intelligence, twoconcentric conductors of cylindrical form, and insulating means forseparating the conductors electrically and maintaining them inconcentric relation, said means comprising spoke-like members ofdielectric material passing trans versely through the inner conductor sothat successive spoke-like members are arranged spirally along theconductor with their outer ends supporting the outerconductor.

3. In a conductive system for the communication of intelligence, twoconcentric conductors of cylindrical form, the inner of said conductorsbeing in the form of a conductive shellv comprising a plurality ofinsulated wires woven together .so that each wire weaves in and outtoward and from the center of the shell, and insulating means forseparating the conductors electrically and mainthe shell, and insulatingmeans for separating the conductors electrically and maintain ing themin concentric relation, said means comprising spoke-like members ofdielectric material passing transversely through the inner conductor sothat successive spoke-like members are arranged spirally along theconductor with their outer ends supporting the outer conductor.

5. In a conductive system for the communication of intelligence, twoconcentric conductors of cylindrical form, both of said conductors beingin the form of a conductive 'shell comprising a plurality of insulatedwires woven together so that each wire weaves in and out toward and fromthe center of the shell, and insulating means for separating theconductors electrically and maintaining them in concentric relation,said means com-.

20 prising spoke-like members of dielectric material passingtransversely through the inner conductor so that successive spoke-likemembers are arranged spirally along the con-' ductor.

6. In a conductive system for the communication of intelligence, twoconcentric conductors of cylindrical form, both of said conductors beingin the form of a conductive shell comprising a plurality of insulatedwires Woven together so that each wire weaves in and out toward and fromthecenter of the shell, and insulating means for separating theconductors electrically and maintaining them in concentric relation,said means.

comprising spoke-like members of dielectric material passingtransversely through the inner conductor so that successive spoke-likemembers are arranged spirally alongthe conductor with their outer outerconductor.

In testimony whereof, I have signed my name to this specification this20th day of May, 1929.

ends supporting the

